The present invention relates to a station selecting method for a television receiver. More particularly, the invention relates to a station selecting method for determining the best frequency for signal reception by using an AFT up signal, an AFT down signal, and a station detection signal.
A station selecting device of the type to which the invention pertains is shown in FIG. 1, which illustrates a television receiver. A signal received through the antenna 1 is amplified by the RF amplifier. The signal thus amplified is converted into an intermediate frequency signal in a frequency mixer 3 by mixing with a local oscillator signal provided by the VCO 4. After being amplified by the IF amplifier 5, the intermediate frequency signal is applied to a video detector 6 with which a video signal is obtained. The video signal thus obtained is applied to the synchronizing signal separator circuit 7 with which a horizontal synchronizing signal and a vertical synchronizing signal are separated from the video signal.
The video carrier is extracted by the video detector 6. The video carrier is applied through a limiter to the AFT detector 8 used to obtain an AFT S-curve output. The S-curve output is applied to the AFT signal generator 9. When the S-curve output signal is lower by at least a predetermined frequency than the center frequency of the video signal, an AFT up signal is produced, and when it is higher by at least the predetermined frequency, an AFT down signal is produced. The output of the synchronizing separator circuit 7 is applied to the station detection signal generator 10 which in response thereto produces a station detection signal (hereinafter referred to as a SD signal) when the output signal of a station is received. These up and down signals and the SD signal are applied to the control circuit 11 which utilizes the SD signal to determine the presence or absence of stations, the AFT up signal to increase the frequency of the local oscillator signal from the VCO 4, and the AFT down signal is used to decrease that frequency.
The forms of the SD signal, the AFT up signal and the AFT down signal are as shown in FIG. 2, in which the predetermined frequency is 2 MHz. In FIG. 2, a period a in which the SD signal is at H (high logic level) and both of the AFT up signal and AFT down signal are at L (low logic level) corresponds to the best receiving frequency range. However, since the same condition is present in the periods b and c in the vicinity of .+-.2 MHz, (where is the center frequency of the video signal), the period b or c can be mistaken for the best receiving frequency range. Especially when there is an offset in the transmitting frequency, it is difficult to correctly detect the correct receiving frequency range period a. This will be described in more detail.
For instance, in receiving television channel 3 (United States specifications) the video frequency (P) is 61.25 MHz and the sound frequency (S) is 65.75 MHz as indicated in FIG. 3. If the local oscillator frequency OSC is 107 MHz, the video frequency (P) and the sound frequency (S) are converted to respective IF frequencies of 45.75 MHz and 41.25 MHz. A SAW filter having a passband characteristic as shown in FIG. 4 passes the channel 3 signal only and suppresses the other unwanted channel signals. The center frequency of the video signal of 45.75 MHz is subjected to FM detection by the AFT detector. If the local oscillator frequency is increased and decreased around 107 MHz, then the S curve signal changes as shown in FIG. 5. It should be noted that FIG. 5 shows the characteristic curve provided when there is no offset in the transmitted channel 3 signal. If adjustment is so made that when the local oscillator frequency is 107 MHz the S curve is at the center (the video carrier is 45.75 MHz) and a window is provided around the center as shown in FIG. 5, then the up signal is produced by the AFT signal generator 9 (a comparator) when the output voltage is above the window (on the left-hand side of the S curve in the drawing) and the down signal is produced when the output voltage is below the window (on the right-hand side of the S-curve). It is accordingly determined whether the local oscillator frequency should be increased or decreased. However, because of the bandpass characteristics of the SAW filter, the upper and lower parts of the S-curve are not symmetrical, and the upper frequency band (the right-hand side of the S-curve) is narrow. Accordingly, in the conventional AFT device utilizing an S-curve detection technique, the width absorbed by the AFT is vertically asymmetrical with respect to the center frequency of the video signal, and it is limited (to about 1 MHz) in upper frequency.